A standard currently proposed by the Advanced Television Systems Committee (ATSC) for advanced television (ATV) transmission in the United States supports two resolutions for high definition transmission: 1920.times.1080 interlace and 1280.times.720 progressive. Due to technology and cost considerations, it is more than likely that practical receiver implementations will be confined to either an interlace or a progressive format. Therefore, for receivers with interlaced displays, a progressive to interlace conversion must be performed whenever 1280.times.720 progressive material must be displayed. Likewise, for receivers with progressive displays, an interlace-to-progressive conversion must be done whenever 1920.times.1080 interlace material must be displayed.
One prior art technique for performing progressive to interlace (P-I) conversion is to sub-sample the image by taking every other line from the progressive frame to generate an interlaced field. However, this technique suffers from the problem that horizontal edges exhibit a high degree of interline flicker. A conventional solution to this problem is to interpolate samples from more than one raster line when generating the output raster. An example of this technique is the so-called "Grand Alliance" Scan Converter, which uses a 6-tap vertical filter and an 8-tap horizontal filter to convert from 1280.times.720 progressive to 1920.times.540 (1 field) interlaced as described in "Grand Alliance HDTV Multi-Format Scan Converter," ICCE Conference, Jun. 7-9, 1995 by B. Bhatt et al. This approach achieves a high quality output image while requiring only a minimal amount of hardware, e.g., no field memories are required.
One prior art method for performing interlace-to-progressive (I-P) conversion, which is also known in the art as: a) progressive scan conversion, b) de-interlacing, or c) sequential scan conversion, is to simply repeat each line vertically, thus doubling the number of lines. This technique is simple and inexpensive to implement. However, this technique is not recommended as the maximum vertical resolution for still scenes is only half of what it otherwise could be.
Another prior art method for performing I-P conversion is to interpolate the missing scan lines of a frame by using a vertical finite impulse response (FIR) interpolator within the current field. Conventionally, this method is not used in high performance systems because it suppresses higher spatial frequencies. Nevertheless, many systems that display video in a window of a personal computer satisfactorily employ vertical interpolation for doing so.
A third method for performing I-P conversion, known as field insertion, consists of combining the odd and even lines of two consecutive fields to generate each frame. This approach suffers from the drawback of 1) generating "mouses teeth" artifacts in areas where there is motion and 2) incurs the cost of a required field memory.
The above-described methods for interlace-to-progressive conversion may be described as one-dimensional in nature. High-end converters add a second dimension, the temporal dimension. To this end, in addition to the information within a single field that is being converted, they also process stored information regarding previous fields as part of the conversion process. Implementations of such spatio-temporal filters require field memories, the particular number required depending on the particular implementation.
In one approach to spatio-temporal filtering, known as motion adaptive inter/intra interpolation, a field is converted to a frame by first assuming that all scan lines in the field will be copied to the respective scan line positions of the frame, and then calculating the missing lines by a) interpolating in the current field if motion is detected, so-called intra-field interpolation; b) inserting a pixel from the previous field if no motion is detected, so-called inter-field insertion; or c) a combination of the two based on the degree of motion. Therefore, a motion calculation has to be made for each pixel location of the missing lines. The downside of this approach is that it may produce annoying artifacts if the motion decision is not correct or if it changes too often due to noise. Also, the subjective quality of the resulting image tends to vary proportionally, within limits, to the number of field memories employed.
The most advanced technique known for I-P conversion is to use motion estimation and motion compensation. The difference between motion detection and motion estimation is that detectors attempt to calculate the motion of a pixel based on the amplitude of its temporal variation, while estimators calculate the actual direction and velocity of objects in motion, e.g., using motion vectors. The advantage of motion estimators is that a very intelligent decision can be made regarding the spatio-temporal interpolation direction. However, accurate real-time motion estimation often implies extensive hardware, and thus high cost.
Unfortunately, there is no single low cost arrangement for providing both I-P and P-I conversion that will produce sufficiently high quality results.